Microelectronic devices generally have a die (i.e., a chip) that includes integrated circuitry having a high density of very small components. In a typical process, a large number of dies are manufactured on a single wafer using many different processes that may be repeated at various stages (e.g., implanting, doping, photolithography, chemical vapor deposition, plasma vapor deposition, plating, planarizing, etching, etc.). The dies typically include an array of very small bond-pads electrically coupled to the integrated circuitry. The bond-pads are the external electrical contacts on the die through which the supply voltage, signals, etc., are transmitted to and from the integrated circuitry. The dies are then separated from one another (i.e., singulated) by cutting the wafer and backgrinding the individual dies. After the dies have been singulated, they are typically “packaged” to couple the bond-pads to a larger array of electrical terminals that can be more easily coupled to the various power supply lines, signal lines and ground lines.
The individual dies can be packaged by electrically coupling the bond-pads on the die to arrays of pins, ball-pads, or other types of electrical terminals, and then encapsulating the die to protect it from environmental factors (e.g., moisture, particulates, static electricity and physical impact). In one application, the bond-pads are coupled to leads of a lead frame, and then the die and a portion of the lead frame is encapsulated in a protective plastic or other material. In other applications for packaging high density components in smaller spaces, the bond-pads are electrically connected to contacts on a thin substrate that has an array of ball-pads. For example, one such application known as “flip-chip” packaging involves placing the active side of the die having the bond-pads downward against the contacts on a ball-grid array substrate, reflowing solder between the contacts and the bond-pads, and then molding an encapsulant around the die without covering the ball-pads on the ball-grid array. Other types of packing that use ball-grid arrays include “chip-on-board,” “board-on-chip,” and “flex-on-chip” devices. These types of devices are generally known as Ball-Grid-Array (BGA) packages.
Many electrical products require packaged microelectronic devices to have an extremely high density of components in a very limited amount of space. The space available for memory devices, processors, displays and other microelectronic components is quite limited in cell phones, personal digital assistants, portable computers and many other products. As such, there is a strong drive to reduce the “footprint” and/or the height of packaged microelectronic devices. This is becoming difficult because high performance devices generally have more bond-pads, which result in larger ball-grid arrays and thus larger footprints. Thus, there is a strong need to reduce the size of BGA packaged devices.
One technique to increase the density of microelectronic devices within a footprint on a printed circuit board is to stack one microelectronic die on top of another. It will be appreciated that stacking the dies increases the density of microelectronic devices within a given footprint. The microelectronic dies are typically stacked on each other in a two-pass process starting with a first pass that mounts a flip-chip die to a substrate and a second pass that mounts a conventional wire-bond die onto the backside of the flip-chip die. The first pass typically involves mounting the flip-chip die to the substrate in a first die attach machine, and then heating the flip-chip/substrate subassembly to reflow solder bumps. The heating process securely attaches the flip-chip to the substrate. After mounting the flip-chip to the substrate, the flip-chip/substrate subassembly is transported to a second die attach machine where it is held for processing in a second-pass. The second pass through the second die attach machine involves (a) dispensing epoxy onto the backside of the flip-chip, and (b) mounting a conventional wire-bond chip to the epoxy. The stacked die assembly is then re-heated to cure the epoxy after the second pass through the second die attach machine.
The conventional two-pass processes for stacking a conventional wire-bond die onto the backside of a flip-chip die typically occur in two different die attach machines. It will be appreciated that a single die attach machine may be used instead by mounting the flip-chip die to the substrate in a first pass, heating the mounted flip-chip in a first heating cycle to reflow solder on the flip-chip die, reprogramming the die attach machine to attach the wire-bond dies to the backside of the mounted flip-chip dies in a separate second-pass through the machine, attaching the wire-bond die to an epoxy on the flip-chip die in a second pass through the same die attach machine, reheating the stacked die assembly, and then reprogramming the die attach machine again to mount flip-chip dies to another set of substrates in a new first pass.
One problem associated with a two-pass procedure for stacking dies is that it inefficiently handles the dies and reduces the throughput of packaged devices. A two-pass system inherently requires a large number of substrates and flip-chip dies to be maintained at the front end of the first-pass die attach machine and a large inventory of flip-chip/substrate subassemblies to be held at the second-pass die attach machine. It will be appreciated that a large number of components are held at various stages of conventional stacking processes, which reduces the efficiency of these processes. Therefore, the conventional two-pass procedures for assembling stacked microelectronic dies are inefficient and reduce the throughput of finished products.
Another problem of two-pass die attach procedures is that they are expensive to implement and operate. For example, in applications that use different die attach machines for the first and second passes, a number of machines are accordingly dedicated to each individual operation and a large number of operators are required to run and monitor the individual machines. It will be appreciated that a significant amount of capital is required for purchasing the machines and building the clean facilities for housing these machines. Moreover, the continuing operating costs for the personnel to operate such a large number of different machines can also be quite high. The two-pass procedures that run two separate passes through a single machine are also expensive because they require a significant amount of down time to reprogram the machine to switch back and forth from processing flip-chip dies to processing conventional wire-bond dies. The significant down time also reduces the throughput and increases the operating cost of using a single die attach machine for performing conventional two-pass procedures.
Although it is desirable to stack dies on each other to form high density microelectronic devices, conventional procedures for stacking the dies on each other are inefficient and costly. Therefore, it would be desirable to develop a more efficient system and method for stacking microelectronic dies in the fabrication of high density microelectronic devices.